The present invention relates to a process for injecting limestone into a furnace to produce a highly reactive lime which is available for downstream flue gas desulfurization processes.
Increased concerns as to the adverse environmental impact of sulfur dioxide emissions and stricter regulations have increased the need for efficient processes for removing sulfur dioxide from the flue gas streams of furnaces utilizing sulfur-containing fuel such as coal or oil. It is well known to utilize lime or hydrated lime to remove sulfur dioxide from furnace flue gas streams.
U.S. Pat. No. 4,609,536 to Yoon et al. discloses a process wherein the lime, utilized as a sorbent, is produced by calcining limestone in a separate reactor. The resultant lime is injected into the flue gas stream downstream of the furnace where it reacts with sulfur dioxide to form solid calcium sulfate and calcium sulfite which is separated from the flue gas stream. The separated solid is innocuous and may be utilized as a construction material or buried in a landfill without adverse environmental effects. As noted in the background of U.S. Pat. No. 5,002,743 to Kokkonen et al., it is also known to inject lime directly into a fluidized bed furnace to reduce the sulfur dioxide content of the flue gas in the furnace.
The Kokkonen '743 patent, U.S. Pat. No. 5,246,364 to Landreth et al. and U.S. Pat. No. 4,788,047 to Hamala et al. disclose two-step processes in which finely ground limestone, calcium carbonate, is injected into a furnace under conditions which result in the calcination of the limestone to lime and the reaction of the resultant lime with sulfur dioxide in the flue gas stream. Due to several factors including the relatively short retention time of the lime in the furnace and inefficient lime utilization in these processes, only a portion of the lime reacts with the sulfur dioxide therein.
Additional sulfur dioxide removal from the flue gas stream may occur in various downstream wet, semi-dry or dry flue gas desulfurization processes. For example, in some processes, the unreacted lime is hydrated in a slurry in a downstream reactor under conditions which favor reactions between the hydrated lime and sulfur dioxide to form calcium sulfite. Hydrated lime is generally more efficient at sulfur dioxide removal than lime which is not hydrated. Alternatively, in semi-dry processes the solid hydrated lime particles are wetted to form a liquid film on the surface of the particle comprising calcium hydroxide in solution which then reacts with dissolved gaseous sulfur dioxide to form calcium sulfite.
Although the basic chemistry of the two-step furnace limestone injection calcination and sulfur removal process appears relatively simple, the overall process chemistry and physics are quite complex. FIG. 2 (derived from "Furnace Dry Sorbent Injection for SO.sub.2 Control Pilot and Bench Scale Studies" prepared for EPRI, Rept. No. 2533-09, November 1992 by Southern Research Institute, Fossil Energy Research Corp. et al.) provides a visual reference for some aspects of the process chemistry and physics.
At elevated temperatures, limestone (calcium carbonate) decomposes to form lime (calcium oxide) and carbon dioxide as represented by the following formula: EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2.
The minimum temperature at which limestone decomposition or calcination occurs in a furnace is dependent upon several factors including the pressure in the furnace and the concentrations of carbon dioxide and water vapor in the combustion gasses. As used herein, the minimum calcination temperature or the calcium carbonate decomposition temperature, generally refers to the temperature at which the rate of limestone calcination is in equilibrium with the rate of recarbonation of the lime to limestone. In conventional furnace applications, the minimum calcination temperature typically ranges between about 1365.degree. Fahrenheit (F.) and 1430.degree. F., but can extend as low as 1200.degree. F., if the furnace operates under negative pressure and the concentration of carbon dioxide in the flue gas is low.
The rate of calcination in the furnace is dependent upon temperature, pressure, carbon dioxide and water vapor concentrations in the furnace gas and the size and quality of the limestone particles injected into the furnace. Smaller particles are heated to their core quicker which increases the overall rate of calcination of the particles. Further, carbon dioxide more readily escapes from smaller particles thereby reducing internal carbon dioxide vapor pressures which would otherwise reduce the rate of reaction.
In the second step of the process, which occurs almost simultaneously with calcination, the resultant lime reacts with sulfur dioxide and oxygen in the flue gas stream to form calcium sulfate as represented by the following formula: EQU CaO+SO.sub.2 +1/2O.sub.2 .fwdarw.CaSO.sub.4.
This reaction may be referred to as quicklime sulfation. The rate at which sulfur dioxide reacts with lime in the furnace is a function of the temperature and pressure in the furnace and the concentration of sulfur dioxide in the flue gas. As used herein, the effective quicklime utilization/sulfation temperature window or envelope refers to the temperature range at which quicklime sulfation occurs at a rate sufficient to result in an appreciable amount of quicklime sulfation in the furnace. The lower end temperature of the effective quicklime utilization/sulfation temperature window, which may also be referred to as the minimum effective quicklime utilization/sulfation temperature, refers to the temperature below which the rate of quicklime sulfation is sufficiently slow to result in a negligible amount of calcium sulfate formation on the particles in the furnace taking into consideration the retention time of the flue gas in the furnace. The relatively short retention time of the flue gas in a pulverized coal type boiler or similar boiler and the relatively high quench rate drives up the minimum effective quicklime utilization/sulfation temperature which typically ranges between about 1600.degree. to 1800.degree. F. The amount of resultant lime conversion typically significantly decreases below about 1800.degree. F. The upper end temperature of the effective quicklime utilization/sulfation temperature window corresponds with the decomposition temperature of calcium sulfate which in a pulverized coal type boiler or similar boiler ranges from about 2,200.degree. to 2300.degree. F.
In existing processes of injecting limestone into a furnace for calcination and quicklime sulfation, the limestone is injected into the furnace at temperatures within the effective quicklime utilization/sulfation temperature window and generally at the higher temperatures thereof all of which exceed the minimum calcination temperature.
Injection of limestone into the furnace for calcination and quicklime sulfation therein provides a relatively inexpensive source of lime for desulfurization, as compared to purchasing and injecting commercially available lime or constructing a separate calcination reactor for providing lime on site. However, the conditions under which the limestone particles are calcined in known furnace limestone injection and calcination processes results in lime particles with reduced reactivity due to sintering, core plugging and complex calcium compound formation from impurities, all of which result in inefficient lime utilization and therefore increased reagent costs and increased downstream auxiliary power requirements.
The efficiency of lime utilization is generally dependent upon the molar percentage of the resultant lime (or calcium ions) which is exposed to sulfur dioxide (i.e. moles of exposed lime divided by the moles of lime). By decreasing the size of the injected limestone particles, and therefore the size of the resultant lime particles, the total surface area, and therefore, the percentage of exposed lime is increased. As carbon dioxide is released from the limestone particles during calcination, pores are formed in the particles which exponentially increase the surface area of each particle and the percentage of exposed lime in the particle. Particle fragmentation during calcination also increases the surface area of the resultant lime.
Sintering, pore plugging and the formation of complex calcium compounds from impurities all significantly reduce the surface area of the lime particles and the percentage of exposed lime in the particle. Sintering involves the loss of surface area and porosity due to physical changes in the particle structure at temperatures below the melting point. Carbon dioxide sintering begins almost instantaneously with calcination. Thermal sintering generally begins at temperatures toward the lower end of the effective quicklime utilization/sulfation temperature window and the rate of sintering increases as the temperature increases.
Pore plugging occurs when calcium sulfate molecules formed on the surface of the lime particle block or occlude the pores thereby significantly reducing the surface area and percentage of lime available for quicklime sulfation. Similarly reactions between lime molecules and impurities in the limestone particle, such as silica, result in the formation of various complex calcium compounds, such as monocalcium and dicalcium silicates, calcium aluminates and dicalcium ferrite. The calcium tied up in these compounds is unavailable for sulfur removal and the formation of these compounds can cause or contribute to pore plugging. The rate of such reactions is generally negligible at temperatures below which quicklime sulfation occurs, but increase as the temperature increases.
In addition to reactions which degrade the quality of the resultant lime at temperatures above the minimum calcination temperature and in particular at temperatures within the effective quicklime utilization/sulfation temperature window, additional reactions which degrade the reactivity of the resulting lime occur at temperatures below the calcination temperature. The extraction of heat from the flue gas cools the flue gas stream rapidly to an exit temperature of approximately 280.degree. to 350.degree. F. As the temperature of the flue gas stream drops below the minimum calcination temperature (1365-1430.degree. F.) the lime will begin to react with carbon dioxide in the flue gas stream to form calcium carbonate (recarbonation), thereby reducing the available lime.
At and below about 900.degree. F., lime will react with water vapor in the flue gas to form solid calcium hydroxide, Ca(OH).sub.2. The solid calcium hydroxide then reacts with gaseous sulfur dioxide to form calcium sulfite as represented by the following equation: EQU Ca(OH).sub.2 +SO.sub.2 .fwdarw.CaSO.sub.3 +H.sub.2 O.
This reaction may be referred to herein as hydrated lime sulfation. Reactions between solid calcium hydroxide and gaseous sulfur dioxide will occur in the furnace at temperatures as low as about 750.degree. F. Below 750.degree. F. the rate of hydrated lime sulfation between solid calcium hydroxide and gaseous sulfur dioxide is sufficiently slow to preclude any significant calcium sulfite formation. Any calcium sulfite formed on the lime particles in the flue gas stream, as it is cooled from approximately 900.degree. to 750.degree. F., reduces the reactivity of the lime particle for downstream processes such as through pore plugging. In addition, water sintering of the lime particle will occur at temperatures as low as about 900.degree. F.
FIG. 3 (derived from "Dry Hydroxide Injection at Economizer Temperatures for Improved SO.sub.2 Control" by Bortz, S. J., Roman, V. P., Yang, P. J., and Offen, G. R., Paper #31 contained in the EPRI "Proceedings: 1986 Joint Symposium on Dry SO.sub.2 and Simultaneous SO.sub.2 /NO.sub.x Control Technologies," vol. 2) provides a graphical representation of the decomposition temperature of relevant calcium compounds in a furnace with the specified concentrations of sulfur dioxide, carbon dioxide and water vapor. The molar concentration of carbon dioxide and water vapor in the combustion gas of typical furnaces ranges between five percent and fifteen percent for each component. Typical sulfur dioxide concentrations range from 1200 to 2300 parts per million by volume. The relatively low minimum effective quicklime utilization/sulfation temperature shown in FIG. 3, approximately 1600.degree. F., is due to the relatively high sulfur dioxide concentration of 3000 parts per million by volume in the combustion gas.
The efficiency of limestone utilization for sulfur dioxide removal may be expressed by the following relationship: EQU U(%)=.DELTA.SO.sub.2 (%)/(Ca/S).
U(%) represents the percent utilization of the calcium content of the injected limestone. .DELTA.SO.sub.2 represents the change in the molar content of sulfur dioxide in the combustion gas. The value of Ca/S represents the molar ratio of the calcium content of the injected limestone to the initial level of sulfur dioxide in the combustion products or flue gas. At a Ca/S ratio of 1.0, the stoichiometric value, the calcium content of injected limestone is theoretically just sufficient to react with all the sulfur dioxide present in the flue gas. However, due to the process inefficiencies as noted above, the calcium utilization is not complete at a Ca/S ratio of 1. The sulfur dioxide removal continues to increase at higher Ca/S ratios and a Ca/S ratio of 2.0 is generally considered to be the economic breakpoint of furnace sorbent injection processes, beyond which the gain in sulfur dioxide removal is not justified by increasing reagent costs. In existing furnace sorbent injection processes, calcium utilization in the furnace generally ranges between approximately ten and thirty-five percent.
The calcium utilization is generally increased in downstream semi-dry and wet desulfurization processes, wherein the partially utilized lime particles are wetted to form calcium hydroxide in a liquid phase (in a liquid layer on the particle in semi-dry processes and in a slurry in wet processes) which reacts with the remaining gaseous sulfur dioxide generally at temperatures of about 30.degree. F. above the water saturation temperature and the water saturation temperature respectively. As noted above, the liquid phase reactions of calcium hydroxide and sulfur dioxide in downstream processes are generally more efficient at sulfur dioxide removal than quicklime sulfation. Unfortunately, the degraded quality of lime provided to such downstream processes by known furnace limestone injection and calcination processes results in inefficient utilization of the calcium content of the lime or limestone feed material.
In summary, in currently utilized processes, limestone is injected into furnaces under conditions which result in calcination of the limestone therein and reactions between the resultant lime and sulfur dioxide in the flue gas to form calcium sulfate in the furnace. Such processes provide a relatively inexpensive source of lime for desulfurization, as compared to purchasing and injecting commercially available lime or constructing a separate calcination reactor for providing lime on site. The goal of such processes is to utilize the heat of the furnace to calcine the limestone to lime while simultaneously achieving sulfur dioxide removal in the furnace. However, the conditions under which the limestone particles are calcined in the furnace in these processes (generally at temperatures within the effective quicklime utilization/sulfation temperature window) result in lime particles with reduced reactivity due to sintering, core plugging and complex calcium compound formation from impurities. These undesirable reactions reduce the percentage of resulting lime available for sulfur dioxide removal thereby decreasing the efficiency of lime utilization and increasing reagent costs and downstream auxiliary power requirements.
There remains a need for processes to provide relatively inexpensive and highly reactive lime which may be efficiently utilized in downstream flue gas desulfurization processes.